Tabular silver halide grains (hereinafter referred to as "tabular grains") containing parallel twinning faces exhibits the following photographic properties:
(1) Tabular grains have a large ratio of surface area to volume (hereinafter referred to as "specific surface area") and therefore can adsorb a large amount of a sensitizing dye by its surface. As a result, tabular grains exhibit a high color-sensitized sensitivity relative to inherent sensitivity.
(2) When an emulsion containing tabular grains is coated on a support and dried, these grains are oriented parallel with the surface of the support. This means that tabular grains can provide a thin coated layer which exhibits excellent sharpness.
(3) In X-ray photograph systems, when a sensitizing dye is added to tabular grains, the absorptivity coefficient of the dye is larger than the absorptivity coefficient of the indirect transition of silver halide (AgX), making it possible to remarkably decrease crossover light. This can prevent deterioration in image quality.
(4) Tabular grains scatter little light and therefore, can provide images having a high resolving power.
(5) Tabular grains have a low sensitivity to blue light. Therefore, if tabular grains are used for a green-sensitive emulsion layer or red-sensitive emulsion layer, a yellow filter can be removed from the emulsion.
Having so many advantages, tabular grains have heretofore been used for high sensitivity commercial light-sensitive materials.
Emulsion grains having an aspect ratio of 8 or more are disclosed in Japanese Patent Application (OPI) Nos. 113926/83, 113927/83, and 113928/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application").
The term "aspect ratio" as used herein means the ratio of diameter to thickness of the tabular grain. The grain diameter can be represented by the diameter of the circle having the same area as the projected area of the grain when the emulsion is observed under a microscope or electron microscope. The grain thickness can be represented by the distance between the two parallel faces constituting the tabular silver halide grains.
U.S. Pat. No. 4,439,520, describes a color photographic light-sensitive material which comprises tabular grains having a thickness of less than 0.3 .mu.m and a diameter of 0.6 .mu.m or more in at least one of green-sensitive emulsion layer and red-sensitive emulsion layer to improve sharpness, sensitivity and graininess.
Tabular grains have a large ratio of surface area to volume. This is advantageous with regard to property (1) noted above. However, this causes a disadvantage in the light-sensitive process. Particularly, electrons produced by the exposure to light migrate in the silver halide grains and are then concentrated on a specific point to form latent images. This phenomenon is called the "concentration principle" in silver halide. This is one of main reasons why silver halide has high photographic sensitivity. However, the distance of travel of these electrons thus produced is finite at normal temperatures and is relatively small. Tabular grains have a wider expansion in the direction parallel with the major faces. Therefore, as compared to isotropic crystals (such as cubic, tetradecahedric and octahedric), electrons need to travel a longer distance in grains having the same volume in order to attain such a concentration effect.
A study of this concentration principle is described in J. W. Mitchell, Quantitative Aspect of the Concentration Theory of Latent Image Formation, Nihon Shashin Gakkaishi, Vol. 48, No. 3, 1985, pp. 191-204. Mitchell suggests that the prevention of dispersion of latent images formed on tabular grains due to such a prolonged electron travel distance may be attained by concentrating electrons on specific points in the tabular grains (e.g., the apexes of the grains, preferably the central portion of the major faces of the grains) to determine the sites for latent image formation.
Furthermore, the concentration of the latent images is a very important factor with regard to the latent image development rate. In general, a chemically-unsensitized emulsion provides a low sensitivity but gives one latent image per one grain. As a result, such an emulsion exhibits a high development rate even at a high intensity exposure which easily causes latent image dispersion. One the other hand, a sulfur-sensitized emulsion has a higher sensitivity but gives a plurality of latent images per one grain (i.e., Poisson distribution). As a result, the development rate is decreased. This phenomenon is caused by the drop in development activity of each latent image due to dispersion of latent images. This fact is described in H. E. Spencer and R. E. Atwell, Journal of Optical Society American, Vol. 54, 1964, pp. 498. It is therefore indispensable that specific points enabling concentration of latent images be formed in the tabular grains in order to assure a high development rate. It goes without saying that the number of these specific points should be as small as possible. Heretofore, a number of techniques have been studied to enable the concentration of latent images. For example, G. C. Farnell, R. B . Flint and J. B. Chanter, "Preferred Sites for Latent Image", Journal Photographic Science, Vol. 13, 1965, pp. 25-31 suggests that these is a close relationship among the sites for latent image formation, structural distortion in grains, and points at which the structural distortion and the grain edge cross each other in tabular silver halide grains having a large size and a high aspect ratio. However, Farnell et al. suggests no method for coordinating this structural distortion in these specific points. Japanese Patent Application (OPI) No. 108526/83 discloses an emulsion of tabular grains having an average aspect ratio of 8 1 or more, characterized in that a silver salt is coordinated on selected positions on parallel opposing (1 1 1) major faces thereof. In this disclosure, the concentration of iodide is controlled between the center of the major face and its surrounding portion so that AgCl is coordinated in the apex or the center of the tabular grains. Furthermore, a site director is adsorbed by the tabular grains so that AgCl is epitaxially coordinated.
This coordination (epitaxy) of AgCl (or other silver salt such as AgSCN) might le effective to restrict the sites for latent image formation. However, such coordination is disadvantageous in that it is subject to change during the subsequent procedures such as rinse, chemical sensitization, coating, and incubation of coated matter due to its high solubility or its tendency to form a mixed crystal with host grains. Therefore, such coordination can hardly maintain its properties.
Japanese Patent Application (OPI) No. 133540/84 discloses a silver halide emulsion containing a silver salt, coordinated epitaxially on selected surface portions on host grains of silver halide having an average aspect ratio of 8:1 or less surrounded by (1 1 1) crystal faces. In this disclosure the host grains do not contain a sufficient amount of iodide to coordinate a silver salt. The coordination of a silver salt is accomplished by allowing a site director to be adsorbed by the host grains.
Japanese Patent Application (OPI) No. 75337/86 discloses a silver halide emulsion containing silver halide grains having a hollow bore portion extending from the surface to the internal portion thereof. However, this disclosure contains no methods for controlling the site for and number of these hollows. Therefore, this approach is not sufficient to concentrate latent images on a small number of specific points.
Japanese Patent Application (OPI) No. 106532/83 discloses a monodispersed emulsion of octahedron or tetradecahedron crystal silver halide grains having an indentation in the center of (1 1 1) faces. Also, J. W. Mitchell, Crystal Imperfection and Chemical Reactivity, Physical Society Bristol Conference, 1954, describes that when a tabular silver bromide grain is etched with a silver halide solvent, etched pits can be specifically etched in the apexes, edges or center of the tabular grains (hexagonal or triangular). In this approach, however, the preparation of tabular grains is accomplished by cooling a saturated solution of silver bromide which has been heated to an elevated temperature. Furthermore, protective colloid such as g&lt;latin are not used. Therefore, this approach cannot be put into practical use in the preparation of a photographic emulsion. Moreover, this approach deals with only silver bromide grains. In this approach, the grain size to be treated is macro size and no consideration is given to the properties of emulsion.
J. W. Mitchell, Quantitative Aspect of the Concentration Theory of Latent Image Formation, Nihon Shashin Gakkaishi, Vol. 48, 1985, pp. 191 -204, describes a process for the preparation of thin tabular grains containing silver iodobromide nucleus in the center thereof, surrounded by a silver bromide phase. J. W. Mitchell also describes that when such tabular grains are treated with a solution of potassium thiocyanate, the grains are dissolved in the center thereof. If this treatment is prolonged, a hole is made in the center of the grains, according to Mitchell. It is believed that the selective dissolution is caused by crystal defect or strain in the center of the grains.
As previously described, tabular grains are thin crystals comprising opposing wide (1 1 1) major faces and having various excellent properties. However, since such tabular grains have widely extended major faces, latent images are dispersed if a larger grain size is used to provide a higher sensitivity. Therefore, even if a larger grain size is used, it is difficult to further improve the sensitivity. In order to solve this problem, it is necessary that the sited for latent image formation be restricted, that the number of the sites for latent image formation be decreased, or that a site by which electrons produced by light can be mostly trapped be selected. Furthermore, such a specific site must be a specific site for a light-sensitive nucleus produced at a chemical sensitization process, as well as a site for effective formation of latent images. Moreover, such a specific site must be stable at each step in the preparation of a silver halide emulsion (e.g., grain formation, desalting, chemical sensitization, coating, and drying) as well as under various conditions imposed after the coating on a film base.
Such a specific site can only be accomplished with tabular grains selectively containing an indentation or space in a specific site thereof, that is, the center of the major faces thereof. This cannot be attained by the prior art approaches.